Methods for treating various types of industrial wastes/sludges are known and have been practiced on a commercial scale. However, attempts to treat mercury-containing wastes/sludges in order reduce the mercury concentrations to environmentally acceptable levels have been scattered, and at best, marginally successful.
For example, U.S. Pat. No. 4,354,942 discloses a method for stabilizing mercury in mercury-containing materials by adding selected inorganic sulfur compounds thereto to prevent the removal or leaching of soluble mercury therefrom. The mercury-containing material is preferred to be pre-treated with a base such as lime or caustic sods. However, the use of inorganic sulfur compounds such as sulfides as disclosed in this patent often results in inadvertent mixing of acids with the sulfides, thereby releasing toxic hydrogen sulfide gas. Even alkaline solutions of certain sulfides emit malodorous vapors. Obviously, this type of treatment method, while effective in reducing the amount of leachable mercury, can cause detrimental effects to the environment.
U.S. Pat. No. 3,804,751 describes a method of disposing waste containing metallic mercury by treating the same with sulfuric acid, followed by neutralizing the treated waste with a basic (lime) slurry to convert the metallic mercury into an insoluble form.
The treatment of mercury in aqueous media by contacting the mercury-containing solution with a sulfide to form insoluble mercury sulfide is disclosed in U.S. Pat. Nos. 3,674,428, 4,147,626 and 4,614,592. However, in the context of mercury-containing wastes/sludges, the addition of a sulfide would cause the same type of toxic and malodorous gaseous vapors mentioned above due to the formation of hydrogen sulfide gas.
Also, methods known as solidification treatment of wastes/sludges by adding cement kiln dust or the like thereto are relatively widespread. Besides cement kiln dust, other calcia-type materials can be used for solidification purposes. See, e.g., R.B. Pojasek, Toxic and Hazardous Waste Disposal, Vol. 1, Ann Arbor Science Publishers, Inc. (1979).
Accordingly, although various attempts have been made in the past to treat mercury-containing wastes/sludges, room for improvement in such treatment methods clearly exists, particularly from an environmental standpoint.